The objectives of this study have been to investigate the modes of action of cationic bactericidal proteins. Efforts have been concentrated on the influence of beta-lysin, a serum protein; arginine-and/or lysine-rich histones; and cetyl-trimethylammonium bromide on three membrane associated enzymes of Bacillus subtilis ATCC 6633 - adenosine triphosphatase, reduced nicotinamide adenine dinucleotide (NADH) oxidase, and NADH-dichlorophenolindophenol (DCIP) and succinic DCIP oxido-reductases. The data obtained thus far reveal that the cationic agents may either stimulate or inhibit the enzymes depending on the system. It is suggested that alterations of the enzymes occur indirectly by combination with regional anionic phospholipid components in the membranes.